I'm unfortunately not there physically to help, but my mom has this light switch she needs to remove temporarily and replace the plate. Her "handyman" installed it 'brand new' within the past few years. I told her it looked like it was from the 80s haha. Anyways, she apparently cannot remove the knob for the life of her, shouldn't the knob be able to simply pull off without breaking?
Usually you pull the knob and it is removed. But being that old it might be stuck or break, but it’s also strange if it was just removed recently. Maybe she needs to pry it on both sides at the same time?
Also tell her to barely tighten those screws. They were too tight and the heat/cold changes was enough to crack it.